RNA interference (RNAi) has become an important tool for reverse functional genomics (Fire et al., 1998). RNAi is a naturally occurring defense mechanism that is highly conserved among eukaryotes. RNAi protects the genome against invention by mobile genetic elements, such as transposons, viruses, and other highly repetitive genomic sequences, and also to control the function of developmental programs in eukaryotic organisms (Sidahmed and Bruce, 2010). RNAi involves the cleavage of double-stranded RNA (dsRNA) by an RNaseIII-type enzyme called Dicer into small interfering RNAs (siRNA), that then direct sequence-specific, homology-dependent, post-transcriptional gene silencing by binding to its complementary RNA and triggering its elimination through degradation or by inducing translational inhibition (Fire et al., 1998; Meister and Tuschl, 2004). RNAi has widely been used to study function, regulation, and expression of gene cascades in both model and non-model insects from a variety of orders including Orthoptera, Dictyoptera, Isoptera, Hemiptera, Coleoptera, Neuroptera, Hymenoptera, Diptera, and Lepidoptera (Belles, 2010). Generally, these studies involve direct injection of dsRNA into various developmental stages. Only a few studies have involved feeding dsRNAs through artificial diets. Delivery of dsRNA by feeding was first demonstrated in 2001 in Caenorhabditis elegans (Timmons et al., 2001), but has since only been documented in a limited number of insect species such as brown apple moth larvae (Epiphyas postvittana; Turner et al., 2006), tsetse fly (Glossina morsitans morsitans; Walshe et al., 2009), termites (Reticulitermes flavipes; Zhou et al, 2008), diamondback moth larvae (Plutella xylostella; Bautista et al., 2009), pea aphid (Aphis pisum), tobacco horn worm (Manduca sexta), and red flour beetle (Tribolium castaneum) (Whyard et al., 2009).
The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte (Coleptera: Chrysomelidae), is perhaps the most destructive insect pest of corn in the US corn belt (Sappington et al, 2006) and the costs of management and crop losses potentially exceed $1 billion (Rice, 2003). Managing corn rootworm has become increasingly difficult because of its sequential evolution of resistance to different insecticide classes (Meinke et al., 1998; Metcalf, 1983; Parimi et al., 2006; Siegfried et al., 2005) and cultural control practices such as crop rotation (Levine et al., 2002; O'Neal et al., 2001). Currently available rootworm management tools include transgenic corn hybrids expressing Bacillus thuringiensis (Bt) toxins such as Cry3Bb1, Cry34Ab1/Cry35Ab1 and mCry3A (United States Environmental Protection Agency 2005a; 2005b; 2006) and seed treatment with neonicotinoid insecticides.
The concept of using RNAi as a method of controlling insect pests is of commercial interest (Baum et al., 2007) and development of an efficient dsRNA delivery by feeding is a prerequisite for using RNAi in crop protection (Price and Gatehouse, 2008). RNAi has been shown to cause larval mortality of WCR in feeding assays using exposure to dsRNA of vacuolar-ATPase (vATPase) subunit A in artificial diet and to transgenic plants expressing dsRNA for this gene which exhibited reduced root injury from larval feeding (Baum et al., 2007). Maintaining WCR larvae on artificial diets (Nowatzki et al., 2006), however, is limited by microbial contamination limiting larval growth to no more than about 7 days. As WCR adults feed extensively on leaf tissues, pollen, tassels, and silk, the effect of dsRNA feeding on WCR adults requires further examination.